Microdisplacement "on" in the middle of a large project
I haven't played around too much myself with microdisplacement, but it sound like what you want to do should work. You will want to make sure you also have adaptive subdivision set to get the best performance. You may still want some of the displacement to come from the displace modifier though, this will require a bit of testing to see what works best.

With remeshing, you may instead want to look at the unsubivide setting on the decimate modifier if the scene was created with applied subdivision modifiers, you will end up with better topology for subdivision.

If viewport performance is the main consideration, you may also want to take a look at the simplify settings under scene properties. It give you the option to control the maximum viewport subdivision for all subdivision modifiers.